1) A low molecule weight protein possessing ADP-ribosyltransferase and NAD glycohydrolase activities and catalyzing the NAD-dependent activation of adenylate cyclase was purified approximately 500-fold from avian erythrocytes. Identification of this enzymatic activity in animal cells suggests that they may activate adenylate cyclase by an NAD-dependent mechanism similar to that used by choleragen. 2) Escherichia coli heat-labile enterotoxin, which is known to catalyze the NAD-dependent activation of adenylate cyclase, was found to possess ADP-ribosyltransferase and NAD glycohydrolase activities. The optimal assay conditions as well as the kinetic constants for the reactants differed from those previously noted in the reactions catalyzed by choleragen. Thus, the two toxins probably activate cyclase by a similar mechanism. 3) Choleragen possesses multiple binding sites for the oligosaccharide derived from ganglioside GM1. Oligosaccharide binding causes changes in its fluorescence and circular dichroic spectra, consistent with the hypothesis that oligosaccharide binding leads to a perturbation of toxin structure. 4) Choleragen activated a partially purified, solubilized adenylate cyclase from brain. Expression of the choleragen-activated cyclase required guanosine-5'-triphosphate and a heat-stable, low molecular weight, calcium-dependent protein. DTL* ZXH-1781-1*